[Radio Wrangler]

Just getting hot could be something as simple as the slider in the quiescent current trimmer losing contact with its track. The quiescent current in the output transistors then goes too high, but not dramatically high and too much heat is created in the output transistors.

The first port of call is to check some voltages: The power supply voltage (67v), the DC level at the positive end of C1L and C1R, and the voltage across each of R124 and R125.

These will give an initial view of what's going on, and show where to go looking next.

You've worked n valve amplifiers before, and in those the valves are normally AC coupled to each other with blocking capacitors between stages. If one valve goes bad, it's DC conditions go bad, but it cannot affect the DC conditions of the other stages. This both localises damage and makes it easier to determine what to replace.

Early transistor designs mimicked valve circuitry and used coupling transformers and DC-isolated stages, but in that era, the germanium transistors gave plenty enough trouble.

Your Quad is one of the first transistor designs to be considered really good. It is a forerunner of the style of design used today. This style uses many transistors and they are all DC coupled together. There are DC blocking capacitors only at the input and the output to allow the Quas to run from a single power rail.

What this means in terms of fault fixing is that a failure of one part can easily damage other devices. If you check parts and replace the first bad one you come across, there is the risk that your new part gets immediately destroyed by whatever killed its predecessor. You of course are now thinking that the part you put in is certainly OK, so you continue onwards, finf the next dud and replace it. It is now killed by the recently deceased part you put in before.

It doesn't always happen, but you can get stuck going round in a loop replacing the same things over and over again. When it does happen, it confuses the living daylights out of people.

So, with transistor amplifiers, the best approach is to identify all the bad parts and to replace the lot of them at once. Sometimes you can't tell if some parts are bad or not, but small parts are cheap enough to swap if there is any doubt about them being OK.

Transistor amplifiers have acquired a reputation for being hard to fix. Yes, they're harder to fix than valve designs, but they still are fixable. They need a different approach, though, and you probably wind up replacing a number of innocent parts. This isn't terrible for cheap parts. In the 1920s, one valve cost a week of someone's wages.

David